HELP, compression problems!

x-jetter

x-jetter

Location
Clarklake, Michigan
I've been breaking in my new Magoo motor this week, and i've run into a problem. I had it out yesterday, ran only about a 1/4 tank through it with no issues, compression was 180 in the front and 175 in the back. Took it out today, drove around for a while, then took the hood off. after taking out the plugs, both holes were smoking just a little bit, but the back was noticably more than the front. so i checked the compression, 180 in the front 175 in the back. Made a very minor adjustment to the low speeds because the plugs were very dry, then took it out for about a half hour more. When i came back in i took the plugs out to see if they were still dry, and the back hole was smoking more than before. checked the compression, 185 in the front and 160 in the back. WTF. i checked the headbolts to make sure they were torqued and everything, and everything was just fine. however, i did notice a gas and oil mixture with some air bubbles where the carb meets the manifold (back carb). they are brand new gaskets, but they must be seated wrong or something. could this possibly be the problem, or am i looking at a much larger problem? could it be something simple like the head orings or something? please tell me this is an easy (cheap) fix, because my wallet is completely tapped from this motor. :frown:
 
Watty

Watty

Random Performance
Location
Australia
I would be taking those carbs off and spraying the gaskets with some hylomar and then re-attaching. I do a leakdown test whenever I have a motor apart and the last 2 times I had engines rebuilt with all new gaskets (intake manifold/carb included) they have leaked from this area.

Granted, this shouldn't effect your compression too much, but certainly won't help the situation.

Do a leakdown test, and check your compression while you're at it. At lease then you will know air isn't escaping somewhere.

Cheers.
 
x-jetter

x-jetter

Location
Clarklake, Michigan
wattage, thanks for the help, a leak down test is gonna be my next option if this doesn't work.

paul read this from atlanta and pmed me with his personal cell number so he could help me out (awesome guy) he said to pull the head and see if there was deto or anything in the back cylinder because of the leak. he's off to dinner now so i don't want to bother him. here's the pics i took, are those vertical 'lines' in the sleeves normal? they appear to be the same in each cylinder, so i don't know if that's normal or not.....
 

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Mouthfulloflake

Mouthfulloflake

ISJWTA member #2
Location
NW Arkansas
those vertical lines look sort of like water ingestion marks?

I would hold off and let Paul give it a clean bill of health.
too much of an investment to rush and do some damage in my opinion.
 
Matt_E

Matt_E

steals hub caps from cars
Site Supporter
Location
at peace
That difference in compression would concern me - did you double and triple check those readings for consistency?
 
Mile9c1

Mile9c1

X-H2O.com
Location
Grand Rapids, MI
You'll see deto marks on the top of your pistons, they show up as tiny holes (because the material actually melts away in the case of detonation). Those piston tops look pretty good to me, granded it's not a big clear picture so that that with a grain of slat. Check the compression when it's cold. Always start off rich when you're figurin' out carbs!
 
Mouthfulloflake

Mouthfulloflake

ISJWTA member #2
Location
NW Arkansas
maybe the headgasket was breached, and letting one cylinder loose a little?
could also explain the water marks.

those are wiseco pistons right?

I broke in a set of them once, it took a long time from what I recall, an hour or so on the trailer with the flush hose to do the initial run in.
 
x-jetter

x-jetter

Location
Clarklake, Michigan
Yea the pistons are still in great shape, and the cylinder walls still have the criss-cross pattern when i run my fingernail over them, so i think everything's good. the orings between the domes and the sleeves were a little melted though. Paul says he thinks the leak in the carb leaned out the mixture, causing the cylinder to overheat, since there's no carbon residue on the back piston. since the mixture was so lean he said there's probably not the right amount of oil in the cylinder, which maybe explains the compression change. i'm gonna pop the head back on and fix the carb gasket tomorrow, then i'll try it again. thanks for the help!
 
x-jetter

x-jetter

Location
Clarklake, Michigan
alright i found the problem today. it looks like the carb gasket that covers the jets/needle/seat assembly in the back carb is bad. it wasn't pressing itself down enough, and there was fuel on the outside of the gasket. i replaced it with one from my 38's and everything appears to be in order. compression is actually better than before, as it is 180 in each hole. my buddies are coming later and we're gonna ride so hopefully i'll be able to get everything dialed in! thanks to everyone (especially paul) for your help!
 
x-jetter

x-jetter

Location
Clarklake, Michigan
if you look closely at the pictures, you'll see that the back piston has virtually no carbon build-up on it. paul said that it probably wasn't getting any fuel so the cylinder basically had no oil in it causing it to have low compression and to overheat and melt the orings, which also may have contributed to low compression.
 
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